1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst that supports catalytic precious metals such as Rh (rhodium), Pt (platinum) and Pd (palladium).
2. Description of the Related Art
Exhaust gas purifying catalysts that purify exhaust gas from automobiles generally includes a catalyst layer formed on the surface of a substrate. The catalyst layer is comprised of a precious metal catalyst such as Rh, Pt and Pd, and a carrier that supports the precious metal catalyst. When exhaust gas passes through the catalyst layer, exhaust gas components are purified on the precious metal catalyst. To effectively optimize catalytic performance of the precious metal catalyst, upstream and downstream catalyst layers may be formed, or a plurality of catalyst layers may be on a substrate.
For example, Example 1 of Published Japanese Translation of PCT application No. 2000-510761 (JP-A-2000-510761) is a catalyst in which an upstream catalyst layer and a downstream catalyst layer are formed upstream and downstream, respectively, in a gas passage. The downstream catalyst layer is further separated into a lower catalyst layer that is formed on the surface of a substrate, and an upper catalyst layer that is formed on the surface of the lower catalyst layer. In Example 1 of JP-A-2000-510761, the upstream catalyst layer supports Pd, the lower catalyst layer of the downstream catalyst layer supports Pd/Pt, and the upper catalyst layer thereof supports Pt/Rh.
When high temperature exhaust gas discharged from an engine passes through the above-described catalyst, however, Rh supported on the upper catalyst layer tends to migrate to the lower catalyst layer or is susceptible to sintering.
Conventionally, CeO2 (ceria) is often added to a catalyst layer. Ceria is able to store oxygen under an oxidizing atmosphere and releases the oxygen under a reducing atmosphere (hereinafter referred to as OSC). Thus, ceria may be used maintain a stable oxygen concentration in exhaust gas passing through the catalyst layer to improve the catalyst activity. In particular, large amount of ceria are often incorporated into the lower catalyst layer, where the oxygen concentration is apt to become low.
However, Rh contained in the upper catalyst layer tends to migrate to ceria in the lower catalyst layer due to the strongly basic nature of ceria. Therefore, when the Rh-containing upper catalyst layer is provided over the ceria-containing lower catalyst layer is exposed to a high temperature atmosphere, Rh is apt to the ceria that is contained in the lower catalyst layer.
The migration of Rh migration results in a lack of Rh in the upper catalyst layer which has a greater contact rate with exhaust gas. Additionally, when Rh is alloyed with the Pt contained in the lower catalyst layer, the purifying performance of both Rh and Pt is reduced.
In this circumstance, Japanese Patent Application Publication No. H06-63403 (JP-A-H06-63403) describes solid-dissolving Ce (cerium) and Nd (neodymium) in ZrO2 (zirconia) to suppress the migration of Rh between layers.
Japanese Patent Application Publication No. 2003-170047 (JP-A-2003-170047) describes supporting Rh and Ba (barium) on a Ce.Zr.Nd composite oxide to suppress sintering of Rh.
Even if cerium and neodymium are solid-dissolved in zirconium oxide as described in JP-H-06-63403, however, it is not possible to sufficiently suppress migration of Rh from the upper catalyst layer to the lower catalyst layer.
The migration of Rh is believed to be due to competition between the force to retain Rh in the upper catalyst layer and the force to attract Rh to the lower catalyst layer. Therefore, optimizing the balance between properties of the upper-layer carrier to retain Rh in the upper catalyst layer and properties of the lower-layer carrier to attract Rh to the lower catalyst layer.
However, when Rh and Ba are supported on a Ce.Zr.Nd composite oxide as proposed in JP-2003-170047, sintering of Rh is not be sufficiently suppressed and, hence, it is not possible to obtain a sufficient purification performance for hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx).